A crystal oscillator has its natural oscillating frequency (vibrating frequency) that is determined by its shape or the like. It is known that this oscillating frequency is expressed by the following equation.
                    f        =                              n                          2              ⁢                                                          ⁢              t                                ·                                    Cij              ρ                                                          [                  Equation          ⁢                                          ⁢          1                ]            
Variables in the equation are as follows:
f: Natural oscillating frequency
n: Overtone order
t: Thickness of crystal oscillator
ρ: Density
Cij: Elastic constant
In general, a crystal oscillator is used as a frequency source in, inter alia, wireless communication equipment and various measuring instruments, as an oscillation element generating a frequency that less changes with temperature and is regarded as the most stable. However, some crystal oscillator has a characteristic in which its oscillating frequency significantly changes with temperature, which is dependent on a cut angle (cut orientation), which is an angle at which it is cut from a crystal material, as typified by Y cut, LC cut, etc.
Taking advantage of such a characteristic of a crystal oscillator, a temperature measurement apparatus is used that employs a crystal oscillator having a cut orientation giving a linear change of frequency with temperature and makes temperature measurement by measuring the oscillating frequency of the crystal oscillator (for example, refer to Patent Literatures 1 and 2). In addition, a quartz temperature measurement device arranged to make temperature measurement by measuring a differential frequency between two crystal oscillators exists (for example, refer to Patent Literature 3). Since the frequency of an electric signal can be measured with high precision, according to such a temperature measurement apparatus using crystal oscillators, it is achievable to make temperature measurement with measurement precision of 1/10000° C. (one ten-thousandth of 1° C.).
By the way, global environmental changes have recently become an issue of concern. Therefore, in order to make a long-term observation of changes in global environment including monitoring crustal movements, a change in seawater temperature, etc., measuring temperature of groundwater, deep wells, crustal heat, deep sea water temperature, etc. is performed by using a quartz temperature measurement device as discussed above. For weather observation and marine weather observation, quartz temperature measurement device as discussed above is also used for an application in which it is installed in the Pacific Ocean to measure seawater temperature in an unmanned manner.
In such a case of measuring deep sea water temperature or the like, although it is possible to observe environmental changes even on the level of measurement precision of 1/10000 ° C. (one ten-thousandth of 1° C.), it is required to measure seawater temperature or the like with even higher precision in order to measure a minute change in temperature.